The present invention relates to methods for the measurement of the tendency of a nonwoven web to curl.
Curl is an important mechanical property of nonwoven webs, such as paper. It is especially important to papers which are used in automated sheet-fed printing operations such as xerography. Excessive curl can cause the paper transport mechanism to jam, thereby creating a great deal of operator frustration, lost time, and service expense. Therefore, there is a need for a measuring device to predict the curl performance of webs which will be used in sheet-fed printing machines.
Existing curl-measuring instrumentation tends to be slow and cumbersome to use. Usually the web is put in an environment which is designed to produce curl, and then the resulting degree of curl is measured.
In one common test, hot-strip-curl (HSC), the curling environment is represented by a curved heated block against which the operator presses four strips of paper to be tested. Upon removal, the arc acquired by a paper strip is compared to a series of arcs marked on a template. The inverse of the radius of curvature of the closest matching arc is noted as the curl value. The values for four strips are combined to produce a differential curl number. Two of the four strips are cut parallel to the machine direction (MD) and two of the strips are cut parallel to the cross-machine direction (CD). For each direction, one strip is held felt side toward the heated block and the other is held wire side to the heated block.
Another common test, split-sheet contraction (SSC) ratio, uses one MD and one CD strip which are each split, using tape, into felt and wire side sections. Following removal of the tape, the resulting four strips are conditioned in a high humidity environment and the length is measured. The humidity is then reduced and the decrease in length is measured. The four length shrinkages are combined into a ratio indicative of curl called the split-sheet contraction ratio.
Operator's skill is required to prepare the sample, verify the suitability of the curling environment, and read the curl value. The reproducibility of the measurement is extremely low, since errors can be introduced at each stage.